This invention relates to a movement for a wrist watch of the type having a large minute disc driven directly at its periphery by the rotor pinion of a stepping motor. Such a movement may be used in a quartz analog watch wherein the stepping motor is driven by an integrated circuit. The minute disc may turn a minute hand on the same sleeve or have a minute indicator painted directly on it.
This type of watch, namely a flat wrist watch, is described in laid open German patent application DE-OS No. 30 16 058. In the case of the known watch, the guide members interacting with the edge of the minute disc to maintain a reliable drive connection are designed as guide rollers to order to keep friction between the guide members and the minute disc as small as possible--in the case of the known watch two superposed minute discs are provided, of which one is transparent. It has become apparent that the results obtained in practice are not entirely satisfactory since the fact that the guide members engage on the edge of a minute disc, which is precision mounted in the customary way, will lead to a considerable strain on the bearing means and therefore to a premature wear and tear of the same. In addition, the very thin minute discs, which are used for particularly flat wrist watches, tend to become distorted and deformed to such an extent that considerable axial forces automatically occur between the guide members and the disc edge when the minute disc is precision mounted. This means that any detrimental effect of these forces cannot be compensated by the fact that the guide members are designed as guide rollers rotatable about an axis parallel to the axis of rotation of the minute disc.
Another watch of this type is known from pending U.S. application Ser. No. 258 061 filed in the name of Paul Wuthrich on Apr. 27, 1981 and assigned to the present assignee, this being also laid open in Germany as DE-OS No. 3214683A1.
Proceeding on the basis of the prior art, the object underlying the invention is to improve a watch of the type described above, such that reliable and low-loss guidance of the disc edge is guaranteed in order to maintain a positive drive connection, even when the minute disc is displaced or deformed axially to a predetermined position of its face, up to the maximum to be expected in practice.
According to the invention this object is accomplished for a watch of the type described above, in that the minute disc is mounted on its pivot pin with extra radial clearance, the decisive advantage of the solution according to the invention is that the minute disc is able to carry out certain swashplate or tilting movements--within predetermined limits, of course--due to the bearing clearance intentionally provided. When the disc is distorted or the disc edge deformed, a simple tilting movement of the entire minute disc, including its bearing sleeve, relative to the pivot pin can be brought about by the guide members, the forces required being extremely low, in order to keep the disc edge in a relatively well-defined position at the point where it is in contact with the drive pinion.
In development of the invention it has proven favorable to have the two guide members designed as guide pins and disposed adjacent to the first main face of the minute disc and to have a third guide pin circumferentially spaced between the other two guide pins and adjacent to the other main face of this disc. This construction has the advantage that, first of all, simple guide pins may be used instead of the relatively complicated guide rollers. These guide pins are formed, in an advantageous development of the invention, simply by extensions of the stepping motor drive means. In addition, this results in a range of tolerance defined by three predetermined points for altering the position of the edge of the minute disc; with a view to practical requirements this range of tolerance may be selected such that the minute disc will normally not engage with any of the guide pins. This becomes clear immediately when it is considered that the thickness of a minute disc is, for example, 0.1 mm, whereas the axial gap, which is defined by the guide members and selected as a function of the height of the drive pinion, for the path of the disc edge may be 0.4 mm and therefore four times the thickness of the disc. On the other hand, even more considerable deformations of the disc edge are not in any way critical because in this case a tilting of the entire minute disc, relative to its pivot pin, will be simply brought about without any critical forces occurring at the disc bearing means, at the guide pins or at the disc edge. This guarantees a long and trouble-free operation of the watch according to the invention with little wear and tear.